FR2908894A1 - OPTICAL PHOTONIC CRYSTAL OPTICAL FILTER - Google Patents

Abstract

The invention relates to an optical filter (1) for reception of an incident laser beam (2), comprisign a non-linear material and able to switch from a transparent to an opaque state depending on the incident beam. The optical filter comprises a 3D photonic crystal (4) in which micro-structures (3) of the non-linear material are arranged, separated by layers of crystal.

Description

1 PHOTONIC CRYSTAL SWITCHABLE OPTICAL FILTER The field of

  the invention is that of the counter-counter optical measurement or CCMO against jamming or destruction caused by optronic countermeasure equipment; it concerns the protection of optronic equipment in the face of the threat of lasers, particularly agile lasers with very short pulse frequencies installed in these countermeasure devices. It is recalled that a frequency-agile laser is a laser able to rapidly change its emission frequencies (typically in a time ranging from microseconds to milliseconds), while maintaining a stable frequency during transmission; it is usually a pulsed laser. The optical bands scanned are typically those of the visible (0.4-0.8 μm), near infrared (0.8-2 μm), and infrared (II-band) (band 3 -5 μm) and band III (band 8-12 pm).

  Current protection technologies can be classified into 4 major groups. - Mechanical shutters: this is the simplest system that can be positioned internally or externally, but is not effective against pulsed lasers. The mechanical control does not allow the mechanical part, the optical density or the absorbent filter to be positioned sufficiently quickly during the aggression: indeed, the positioning time is greater than one millisecond. - Fixed filters: they use the reflection or absorption properties of the materials used and are centered around a wavelength. They are usable against pulsed or continuous lasers. The interference filters that form the most used concept for rejecting unwanted wavelengths are commonly constituted by multilayers of dielectric materials deposited on a substrate. A low tolerance to the angle of incidence of the threat and the impact of these filters on the optical performance of the optronic system present significant technical limitations. In addition, these filters can not meet the need for protection against frequency-agile lasers. 2 Switchable filters that use the reflection or absorption properties of active materials with controllable optical bandwidth over time. They are usable against continuous or long pulsed lasers. The materials can be electro-active.

The activation time is of the order of the micro-milli-second, which does not make it possible to respond correctly to the need for protection against frequency-agile lasers. - Active or passive optical limiters. Active limiters are triggered by an external power source, such as an electrical voltage. It is thus possible to construct optical valves with a mixture of liquid crystal polymer composites (PDLC or Polymer Dispersed Liquid Crystal). The PDLC mixture is homogeneous and transparent when the control voltage is applied: indeed this control voltage allows the alignment of the liquid crystal beads. When the tension is canceled, the balls randomly arrange themselves in the structure and the material becomes diffusive. The system thus passes from an optical on state to an optical blocking state. The response time is limited by the viscosity of the liquid crystal, which corresponds to the rotations of the molecules.

The so-called passive limiters are triggered by the laser pulse, the trigger then depending on the laser power; other limiting concepts are related to changes in the refractive index of the material due to the rise in internal temperature. Nonlinear effects can occur in liquids, solids and gases. The exploitation of these effects makes it possible to design the different limiters. It will be recalled that the various nonlinear effects are described by a Taylor series development of the optical polarizability P as a function of the electric field E. The terms of the series represent the susceptibility tensors. The first order susceptibility tensor is related to the linear optical domain of the material, to the linear index of refraction, to the linear absorption. The second order of the susceptibility tensor includes refractive, electro-optical photo effects. The third order describes two-photon absorption, saturable absorption. Two-photon absorption (ADP) is a third-order effect in which there is a transition from the basic state to the excited state by simultaneous absorption of two photons of frequency co. Absorption a can be written in this case a = a o +, 3o 1 where / 3o is the ADP coefficient, ao is the linear absorption coefficient of the considered material and I is the irradiance (in Wm- 2) associated with the electromagnetic field. In addition, recent work has shown that the non-linear absorption phenomenon can be exalted by ADP-induced excited state absorption processes; these effects are observed at high flux, and it has been established that the term of reabsorption in the excited state is a preponderant phenomenon in the non-linear absorption.

The effectiveness of these limiters is naturally related to the trigger point of the nonlinear material. To reach these trigger thresholds, it is often necessary to position these limiters in the intermediate focal planes in order to artificially increase the focus of the laser beam and thus to increase the energy densities per unit area. Nonlinear materials in homogeneous layers or in suspension in solutions have already been studied. In the Visible / PIR band, the following suspended materials: CBS (Carbon Black Suspension) Porphyrin PDLC Carbon black suspensions (CBS) have a limited stability which is related to its linear transmission. The dilution necessary to obtain a clear solution involves a high dilution, and this reduces the stability of the solution. In IR band II, the following layered materials: InAs, VO2, HgCdTe. In IR III band, the following layered materials: InSb, VO2. The current limiters based on these materials also require the limiters to be positioned in an intermediate focal plane, with very high intensities to switch from the on state to the off state, which implies the addition of expensive and bulky optics. in the devices to be protected. The protections are rarely broadband, are sensitive to the angle of incidence of the threat (hence the need to place the limiter in an intermediate focal plane) and have insufficient contrast between the passing state (transparent). ) and the blocking state (opaque): the contrast of the transmission coefficient is for example equal to 60 / 0.05; this contrast is given as a percentage of transmission. The object of the invention is to provide an optical filter not having the aforementioned drawbacks. In order to improve these limiters or nonlinear filters, photonic crystals are used: these artificial crystals make it possible to artificially modify the optical properties of the nonlinear materials which constitute these limiters. More specifically, the invention relates to an optical filter for receiving an incident laser beam, comprising a non-linear material and able to switch from a transparent state to an opaque state depending on the incident beam. It is mainly characterized in that it comprises a 3D photonic crystal in which micro-structures of the nonlinear material are arranged, separated by crystal layers. Preferably, the crystal having a front face intended to receive the incident beam and a rear face, the size of the micro-structures is increasing for example linearly and the distance separating them decreasing when going from the front face to the face back. The distance between the centers of the micro-structures is for example invariant. Advantageously, the micro-structures are arranged in the crystal in a network which may be a network of cubic or hexagonal meshes. According to one embodiment of the invention, the nonlinear material is VO2 and the crystal is Sapphire. Typically, the microstructures are arranged in 5 to 100 layers, the size of the microstructures is between 40 nm and 300 nm and the grating has a pitch of between 100 nm and 2 μm. According to one characteristic of the invention, the filter has a bandwidth of between 3 and 12 μm and more precisely between 3 and 5 μm. The invention also relates to an optronic system comprising such a filter advantageously located in the focal plane.

This system may be an optronic sensor such as a thermal camera. The invention also relates to a method of optical filtering a frequency-agile laser beam by means of such a filter.

According to one characteristic of the invention, the filter is able to switch in less than 10 ns. According to another characteristic of the invention, the laser is a pulsed laser. The laser typically has a wavelength of between 3 and 12 μm. Other characteristics and advantages of the invention will appear on reading the detailed description which follows, given by way of nonlimiting example and with reference to the appended drawings, in which: FIG. 1 schematically represents an example of a filter according to FIG. FIGS. 2a and 2b schematically represent meshes of the photonic crystal used in the invention, FIG. 3 diagrammatically represents an example of an optronic sensor 20 comprising a filter according to the invention. It is recalled that VO2 is a vanadium oxide which has two distinct crystalline states. Below 67 C the crystalline cell is monoclinic and corresponds to a semiconductor state. Above 67 C the crystalline mesh becomes tetragonal and corresponds to a metallic state. The optical properties of the two states are very different. We go from a low absorption state (refractive index of 3, absorption coefficient k of 0.1) to a very absorbing state (index of 3, absorption k of 6) by a very fast phase change (lower at 120 femtoseconds) controlled by temperature. It is neither a two-photon absorption nor a classic third-order non-linearity but a super-fast thermal effect: a non-linear thermal effect.

It is used for optical limitations uses in band II and III, the laser impact heating the material and moving it from the transparent state to the opaque state. This type of material, however, suffers from two major defects.

Firstly, to obtain sufficient attenuation in the opaque state, it is necessary to put a thickness of material such that the transmission in the transparent state is strongly degraded: less than 60% of transmission in the transparent state to guarantee to be transparent. below 5% in the opaque state. Secondly, the phase transition of VO2 is not rapid (i.e., less than 10ns) than on the surface layers of VO2. Indeed, the massive VO2 illuminated by a laser flash only switches from the semiconductor state to the metallic state on a surface layer of 200 nm. All the VO2 located at more than 200 nm deep does not contribute anything to the phase transition. The study of the field map inside the structure 15 shows that the VO2 layers switch in an order related to the intensity of the field. The VO2 patch on the brightest area of the field map rocks first and prevents deeper layers from tipping over in turn. The practical attenuation value of the limiter is thus limited to that produced by this thin layer which is too weak to be useful in a practical case (switching of the transparency from 80% to 20%). According to the invention, the photonic crystal makes it possible to modify the properties of the non-linear material such as VO2 and its response to laser illumination.

The invention is based on the use of the structure of photonic crystals to organize micro-structures of VO2, called patches, to give the set of nonlinear optical qualities that do not have massive VO2 nor the VO2 deposited in thin layers as recalled above. More specifically, the switchable optical filter 1 according to the invention shown in FIG. 1 comprises a 3D photonic crystal 4 in which microstructures 3 of the nonlinear material are disposed, separated by crystal layers. According to a first mode of operation corresponding to that of FIG. 1, the microstructures of VO2 are arranged in three dimensions (3D), in cubic mesh, separated by transparent layers of photonic crystal in band II or in band III . The distance 32 between the centers of micro-structures is invariant. For the band II, the photonic crystal is for example sapphire. These microstructures must be small in front of the wavelengths envisaged and the fill rate (the amount of VO 2 contained per unit volume) very low, of the order of a few%. This point makes it possible to obtain high opacity in the off state without sacrificing the transparency in the on state. The size of the VO2 microstructures as well as their spacing must vary slowly from one layer to another and in keeping with a precise profile: the VO2 patches 3 have an increasing size as one goes deeper and deeper. in the structure, from the face of the crystal intended to receive the incident beam 2, and the gap 31 between the layers of VO2 patches is reduced as one sinks into the filter structure .

Compliance with these conditions makes it possible to obtain the following properties: the transmission curve of the filter is substantially uniform in the spectral band under consideration, the interference pattern created by the filter is of increasing intensity in the patches of FIG. VO2 as you progress to the buried layers. The structure of the filter according to the invention produces a field map where the maximum light intensity is on the deepest layer, the immediately lower intensity is on the penultimate layer, and so on. In this way, the deep layers of VO2 switch from the semiconductor state to the metallic state before the surface layers. We have a cascading effect where the most buried layer changes state just before the one immediately preceding it, then, by domino effect, all the layers switch in the metallic state from the bottom to the surface of the structure. The phenomenon is also very fast: the entire structure 30 switches in less than 10ns. The filter structure has a characteristic peculiar to all photonic crystals: its spectral behavior is proportional to its size. This amounts to saying that if one multiplies all the dimensions by two, the spectral response is identical but for wavelengths multiplied by two.

In the case of the 3-5pm band, there are for example patch sizes varying between 40 and 300 nm, a mesh pitch 32 of the array varying between 100 nm and 2 μm, a number of layers ranging from 5 to 100. A filter whose evolution of microstructure size is linear and whose microstructure size ratio between the first and the last layer of VO 2 is 0.4 gives good results. Such a filter comprises between five and ten layers. In another example, the square-meshed photonic crystal, of pitch 32 of 300 nm, comprises 30 layers separated by a dielectric such as sapphire whose distance between patches of VO 2 is nominally 200 nm but varies slowly in a layer. to the other because the size of the patches decreases. Thanks to this reduction in size, the transmission curves do not contain oscillations or ripples. The organization of these microstructures previously described makes it possible to obtain pass-phase transparencies greater than those of bulk or thin-film VO 2 without reducing the opacity in the blocking phase. The dynamics of the optical limiter is improved: for the example cited, we obtain transmission coefficients> 90 / <10 "6 instead of 60 / 0.05.

Other 3D network arrangement techniques corresponding to other modes of operation may be used. It is possible to have: a lattice of centered cubic lattice such as that described in FIG. 2a, a non cubic mesh such as, for example, a hexagonal mesh such as that described in FIG. 2b, or even other mesh shapes, or else a profile of variation patch sizes that are no longer linear. In these figures 2a and 2b, are represented the centers of the 3 microstructures of VO2 separated by crystal (or dielectric material) 4, and the edges (or no mesh) 32 connecting two centers of microstructures. Their growing size is not represented. The examples of mesh of these figures comprise 3 layers of VO2 located in the following planes: two opposed square faces and a parallel face midway away from the preceding ones for the centered cubic mesh of FIG. 2a, two opposed hexagonal faces and a face parallel to the half of the preceding for the hexagonal mesh of Figure 2b. This type of photonic crystal organization allows the use of the entire thickness of the material and not only the surface layer 10 during the impact of a laser pulse. This allows the structure to retain a high contrast passing / blocking even in front of a pulse and thus to protect itself, which the use of massive V02 can not ensure because its useful thickness remains limited to 200nm. The time constants are small and result in high switching speed. The structure switches from the state to the blocking state in less than 10ns (5ns most of the time for an illumination of 600J / m2) during a laser impact. This makes it possible to protect against laser pulses of this order of magnitude. Other structures of VO2, massive or not, do not allow it.

By analogy with the behavior of all other photonic crystals, the structure has some independence from the angle of incidence in contrast to a thin-film structure. The usable spectral band is wide and makes it possible to respond to a threat of a frequency agile laser. The structure used with V02 25 makes it possible to cover the band II and the band III. The structure remains valid for any wavelength by simple scale change, only the choice of the optical material including the patches can oppose a use from the visible to the far infrared. The obtained filter can be placed in the focal plane, the intermediate focal plane becoming useless. An example is VO2 for use in bands II and III. This invention is applicable to other types of materials having equivalent properties in different optical bands, such as nanoaggregates of metal insulating phase transition materials, perovskite structure materials such as rare earth nickelates. The filter according to the invention may be installed in an optronic system that may be subjected to laser attack from, for example, decoy devices or optronic countermeasures. This system is typically an optronic sensor including a thermal camera, directly or indirectly measuring the heat produced by the incident radiation to be detected. The spectral range of sensitivity of such a thermal camera is located in the infrared.

FIG. 3 shows an example of an optronic sensor comprising a lens 200 and a filter 1 according to the invention located at the focal length F of the lens and intended to receive an incident beam 2. The filter is disposed on the detector 100 which comprises a focal plane 101, a read circuit 102 and a detection chip 103.

The filter according to the invention comprises a set of layers comprising periodic networks of micro-structures of the non-linear material in question (VO2 for example) and transparent dielectric materials (sapphire for example). This type of filter can be placed near the focal plane of detection as in the example of FIG.

One potential technology for manufacturing this photonic crystal is to fabricate each crystal plane independently and then assemble the photonic crystal. Each plane of the crystal consists of a layer of dielectric material (sapphire for example) on which the microstructures of non-linear material are directly manufactured. Four major steps are required in this manufacturing process. The first step is the deposition of a resin on the sapphire substrate, for example), the second step is lithography, which consists of transferring the microstructure pattern onto the resin, a direct lithography technique being for example beam writing. electron.

This technique is the most used for reasons of small dimensions (typically of a few tens of nanometers). Once the part of the resin attacked by the lithography, it is then necessary to etch the microstructures in the substrate following the pattern inscribed in the resin, which is the third step of the process. The most suitable type of process for submicron pattern etching is plasma etching (Plasma Etching), or RIE (Reactive Ion Etching) type etching. The last step involves depositing the non-linear material and removing the resin. The material fills the etched microstructures and covers the untouched resin; it remains only to remove the untouched resin, which is achieved in practice by for example dipping the substrate in acetone. The process is replicated for each plane of the photonic crystal; it is then sufficient to assemble the different layers obtained to obtain the photonic crystal.

Claims (18)

  An optical filter (1) for receiving an incident laser beam (2), comprising a non-linear material and able to switch from a transparent state to an opaque state as a function of the incident beam, characterized in that it comprises a photonic crystal (4) 3D in which micro-structures (3) of the non-linear material are arranged, separated by crystal layers.   2. Optical filter according to the preceding claim, characterized in that the crystal having a front face (41) intended to receive the incident beam and a rear face (42), the size of the microstructures is increasing and the distance (31) separating them decreasing when we go from the front to the back side.   3. Optical filter according to the preceding claim, characterized in that the distance (32) between the centers of micro-structures is invariant.   4. Optical filter according to one of claims 2 or 3, characterized in that the size of micro-structures increases linearly.   5. Optical filter according to one of the preceding claims, characterized in that the micro-structures are arranged in the crystal in a network.   6. Optical filter according to the preceding claim, characterized in that the network is a network of cubic or hexagonal meshes.   7. Optical filter according to one of the preceding claims, characterized in that the nonlinear material is VO2 and the crystal is Sapphire.   8. Optical filter according to one of claims 5 to 7, characterized in that the microstructures are arranged in 5 to 100 layers, the size of microstructures is between 40 nm and 300 nm and the network has a step between 100 nm and 2 pm.   9. Optical filter according to one of the preceding claims, characterized in that the filter has a bandwidth of between 3 and 12 μm.   10. Optical filter according to the preceding claim, characterized in that it has a bandwidth in the band 3-5 pm.   11. Optronic system comprising a filter (1) according to one of the preceding claims.   12. System according to the preceding claim comprising a focal plane (101), characterized in that the filter (1) is located on the focal plane.   13. System according to one of claims 11 and 12, characterized in that the system is an optronic sensor. 20   14. System according to the preceding claim, characterized in that the optronic sensor is a thermal camera.   15. A method of optical filtering a frequency-agile laser beam by means of a filter (1) according to one of claims 1 to 10.   16. Optical filtering method according to the preceding claim, characterized in that the filter is able to switch in less than 10 ns.   17. Optical filtering method according to one of claims 15 or 16, characterized in that the laser (2) is a pulsed laser.   18. An optical filtering method according to one of claims 15 to 17, characterized in that the laser (2) has a wavelength of between 3 and 12 pm. 10 25